Wing flex at higher speed DG-100

I noticed for the first time when flying at speeds above 200 km/h, my wingtips start to bend down a bit, has anyone ever noticed this? Glider is a DG-100.

Yes.

I flew a DG-100 at the F1GP. Approaching redline the downward bend was impressive.

On Wednesday, September 11, 2019 at 10:00:12 AM UTC-4, Senna Van den Bosch wrote:
I noticed for the first time when flying at speeds above 200 km/h, my wingtips start to bend down a bit, has anyone ever noticed this? Glider is a DG-100.

Yep. The twist that helps provide the very docile handling hurts at higher speeds.
The airfoil is not very efficient above 150 or so. Not much reason to fly very fast.
FWIW
UH

On 9/11/2019 8:11 AM, wrote:
On Wednesday, September 11, 2019 at 10:00:12 AM UTC-4, Senna Van den Bosch
wrote:
I noticed for the first time when flying at speeds above 200 km/h, my
wingtips start to bend down a bit, has anyone ever noticed this? Glider
is a DG-100.

Yep. The twist that helps provide the very docile handling hurts at higher
speeds. The airfoil is not very efficient above 150 or so. Not much reason
to fly very fast. FWIW UH

"What UH said."

Me knowing zero/zip/nada about the DG-100's airfoil choice(s?), reason for the
downward-bend-phenomenon at higher speeds lies in some flavor of
geometric/aerodynamic twist from root to tip, where - in essence - the tip
airfoil is at a lower/reduced angle of incidence (and hence angle of attack)
to the root airfoil...done for the reason UH mentions, i.e. to ensure the root
stalls prior to the tip, helping ensure low-speed behavior is benign. None of
this "sudden tip stalling" excitement to worry about. Twist is a
less-graceless manner of avoiding (say) stall strips often seen inboard on
power planes (e.g. Bonanzas).

At higher speeds (lowering AoA), reversed aerodynamic effects become apparent,
as you've now seen. The tip will reach negative AoA prior to the root. Cool!

If the reason is geometric twist from root-to-tip (i.e. the same airfoil),
sometimes you can actually *see* that by gazing from tip to root along the
apar. Kinda cool, for the wannabe aerodynamicist/designer in you!

Bob W.

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On Wednesday, September 11, 2019 at 10:11:06 AM UTC-4, wrote:
On Wednesday, September 11, 2019 at 10:00:12 AM UTC-4, Senna Van den Bosch wrote:
I noticed for the first time when flying at speeds above 200 km/h, my wingtips start to bend down a bit, has anyone ever noticed this? Glider is a DG-100.

Yep. The twist that helps provide the very docile handling hurts at higher speeds.
The airfoil is not very efficient above 150 or so. Not much reason to fly very fast.
FWIW
UH

Yup. The wing twists because of inadequate torsional stiffness.
IIRC, as you go faster, the center of lift moves aft, so twists the wing.
And as it twists, it aggravates the situation by further reducing tip AOA.
Later designs use a variety of trickery to minimize this effect.
Carbon wing skins (stiffer) help, hence big reduction in twist from
LS6a to LS6b with corresponding high-speed performance improvement.

Recent designs (eg from Boermans, Antares and later) use tailored airfoils
rather than twist to obtain gentle stall and minimize this.
On open and higher-aspect-ration ships this is quite challenging!
Concordia design worked hard to minimize twist (Dillinger's Phd thesis).
Don't know about Nexis.

Interesting stuff...
See ya, Dave

Op woensdag 11 september 2019 16:54:07 UTC+2 schreef BobW:
On 9/11/2019 8:11 AM, wrote:
On Wednesday, September 11, 2019 at 10:00:12 AM UTC-4, Senna Van den Bosch
wrote:
I noticed for the first time when flying at speeds above 200 km/h, my
wingtips start to bend down a bit, has anyone ever noticed this? Glider
is a DG-100.

Yep. The twist that helps provide the very docile handling hurts at higher
speeds. The airfoil is not very efficient above 150 or so. Not much reason
to fly very fast. FWIW UH

"What UH said."

Me knowing zero/zip/nada about the DG-100's airfoil choice(s?), reason for the
downward-bend-phenomenon at higher speeds lies in some flavor of
geometric/aerodynamic twist from root to tip, where - in essence - the tip
airfoil is at a lower/reduced angle of incidence (and hence angle of attack)
to the root airfoil...done for the reason UH mentions, i.e. to ensure the root
stalls prior to the tip, helping ensure low-speed behavior is benign. None of
this "sudden tip stalling" excitement to worry about. Twist is a
less-graceless manner of avoiding (say) stall strips often seen inboard on
power planes (e.g. Bonanzas).

At higher speeds (lowering AoA), reversed aerodynamic effects become apparent,
as you've now seen. The tip will reach negative AoA prior to the root. Cool!

If the reason is geometric twist from root-to-tip (i.e. the same airfoil),
sometimes you can actually *see* that by gazing from tip to root along the
apar. Kinda cool, for the wannabe aerodynamicist/designer in you!

Bob W.

---
This email has been checked for viruses by AVG.
https://www.avg.com

Wow, never knew about this until now, I could actually see a nice curve going from the root of the wing to the tip, pretty cool to see, but had me a little worried (220 kph and I see that for the first time). I usually don't fly above 150, however that was fun to pass alongside the airfield at low altitude at the end of the day

On Wednesday, September 11, 2019 at 7:11:06 AM UTC-7, wrote:
On Wednesday, September 11, 2019 at 10:00:12 AM UTC-4, Senna Van den Bosch wrote:
I noticed for the first time when flying at speeds above 200 km/h, my wingtips start to bend down a bit, has anyone ever noticed this? Glider is a DG-100.

Yep. The twist that helps provide the very docile handling hurts at higher speeds.
The airfoil is not very efficient above 150 or so. Not much reason to fly very fast.
FWIW
UH

What always impressed me was the up down flex of my Nimbus 4 wings. Especially in Sierra turbulence those wing tips must have been bouncing 12 feet.

On Wednesday, September 11, 2019 at 9:54:32 AM UTC-7, Senna Van den Bosch wrote:

Wow, never knew about this until now, I could actually see a nice curve going from the root of the wing to the tip, pretty cool to see, but had me a little worried (220 kph and I see that for the first time). I usually don't fly above 150, however that was fun to pass alongside the airfield at low altitude at the end of the day

The LS4 was the same. The washout that makes it so nice to fly at normal speeds does that.
The best fix is... Don't look at the tips near red line.
Jim

Op woensdag 11 september 2019 19:54:37 UTC+2 schreef JS:
On Wednesday, September 11, 2019 at 9:54:32 AM UTC-7, Senna Van den Bosch wrote:

Wow, never knew about this until now, I could actually see a nice curve going from the root of the wing to the tip, pretty cool to see, but had me a little worried (220 kph and I see that for the first time). I usually don't fly above 150, however that was fun to pass alongside the airfield at low altitude at the end of the day

The LS4 was the same. The washout that makes it so nice to fly at normal speeds does that.
The best fix is... Don't look at the tips near red line.
Jim

I still have the standard tips, which have a downward skid, which are the same as you can see on this pictu http://aviation.derosaweb.net/dg101/...1RP_deploy.JPG
Would having the optional TN 301/20 wingtips/winglets fitted make a difference to this downward flexing?

On Wednesday, September 11, 2019 at 1:16:02 PM UTC-5, Senna Van den Bosch wrote:

I still have the standard tips, which have a downward skid, which are the same as you can see on this pictu http://aviation.derosaweb.net/dg101/...1RP_deploy.JPG
Would having the optional TN 301/20 wingtips/winglets fitted make a difference to this downward flexing?

No. Any small change in the last 10-12 inches will not change the twist that has been imparted of the last 10 feet of the wing.

Steve Leonard